1. Field of the Invention
The present invention relates to a thermal recording process for recording a gradation image on a thermal recording medium with a laser beam.
2. Description of the Related Art
Thermal recording apparatus for applying thermal energy to a thermosensitive recording medium to record an image or other information thereon are in wide use. Particularly, thermal recording apparatuses which employ a laser output source as a thermal energy source for high-speed recording are known from Japanese laid-open patent publications Nos. 50-23617, 58-94494, 62-77983, and 62-78964, for example.
The applicant has developed a thermal recording medium capable of recording a high-quality image for use in such thermal recording apparatus. The thermosensitive recording medium comprises a support base coated with a coloring agent, a color developer, and light-absorbing dyes (photothermal converting agent), and produces a color whose density depends on the thermal energy that is applied to the thermosensitive recording medium. For details, reference should be made to Japanese laid-open patent publications Nos. 5-301447 and 5-24219.
The thermosensitive recording medium has a thermosensitive recording layer on the support. The thermosensitive recording layer is produced by coating a coating solution on the support base. The coating solution contains an emulsion which is prepared by dissolving microcapsules containing at least a basic dye precursor, a color developer outside of the microcapsules, and light-absorbing dyes outside of the microcapsules into an organic solvent that is either slightly water-soluble or water-insoluble, and then emulsifying and dispersing the dissolved materials.
The basic dye precursor produces a color by donating electrons or accepting protons as of an acid or the like. The basic dye precursor comprises a compound which is normally substantially colorless and has a partial skeleton of lactone, lactam, sultone, spiropyran, ester, amide, or the like, which can be split or cleaved upon contact with the color developer. Specifically, the compound may be crystal violet lactone, benzoil leucomethylene blue, malachite green lactone, rhodamine B lactam, 1,3,3-trimethyl-6'-ethyl-8'-butoxyindolino-benzosplropyran, or the like.
The color developer may be of an acid substance such as a phenolic compound, an organic acid or its metal salt, oxybenzoate, or the like. The color developer should preferably have a melting point ranging from 50.degree. C. to 250.degree. C. Particularly, it should be of a slightly water-soluble phenol or organic acid having a melting point ranging from 60.degree. C. to 200.degree. C. Specific examples of the color developer are disclosed in Japanese laid-open patent publication No. 61-291183.
The light-absorbing dyes should preferably comprise dyes which absorb less light in a visible spectral range and have a particularly high rate of absorption of radiation wavelengths in an infrared spectral range. Examples of such dyes are cyanine dyes, phthalocyanine dyes, pyrylium and thiopyrylium dyes, azulenium dyes, squarylium dyes, metal complex dyes containing Ni, Cr, etc., naphtoquinone and anthraquinne dyes, indophenol dyes, indoaniline dyes, triphenylmethane dyes, triallylmethane dyes, aminium and diimmonium dyes, nitroso compounds, etc. Of these dye materials, those which have a high radiation absorption rate in a near-infrared spectral range whose wavelength ranges from 700 nm to 900 nm are particularly preferable in view of the fact that practical semiconductor lasers have been developed for generating near-infrared laser radiation.
In order to keep the thermosensitive recording medium in stable storage, the thermosensitive recording medium is designed such that it does not produce a color at a thermal energy which level is lower than a certain threshold value. Therefore, the laser output source is required to produce a considerable level of thermal energy for enabling the thermosensitive recording medium to produce a desired color. The thermosensitive recording medium may be scanned with a laser beam at a low speed to apply a sufficient level of light energy for thereby generating a sufficient level of thermal energy. However, the low-speed scanning lowers the recording efficiency. In addition, an increase in the laser output power for increasing the level of thermal energy will increase the cost of the thermal recording apparatus.
The thermosensitive recording medium tends to suffer thickness irregularities of the thermosensitive recording layer in the manufacturing process, and such thickness irregularities are responsible for irregularities in recorded images which cannot be ignored. While this draw-back can be alleviated to some extent by increasing the accuracy with which to manufacture the thermosensitive recording medium, any required expenditure of time and money will be prohibitively large.